1. Field of the Invention
The present invention relates to automatic transmissions of a type having a hydraulically operated friction element and a hydraulically operated band brake, and more particularly to automatic transmissions of a type having a hydraulic servo device for the band brake.
2. Description of the Prior Art
In order to clarify the task of the present invention, a conventional automatic transmission having a hydraulic servo device for a band brake will be briefly described in the following, which is disclosed in Japanese Patent First Provisional Publication No. 63-251652.
A band brake is employed in the disclosed transmission, which is released when the transmission assumes first or third gear and applied when the transmission assumes second or fourth gear. For selectively releasing and applying the band brake, a hydraulic servo device is employed, which uses two pistons. One piston is used for applying the band brake for second gear and the other piston is used for applying the band brake for fourth gear. That is, when the piston for second gear is applied at one surface thereof with a certain hydraulic pressure, the band brake is applied to establish second gear. When shifting from second gear to third gear is desired, the piston for second gear is applied at the other surface thereof with a certain hydraulic pressure (viz., band brake releasing pressure) with the pressure for second gear kept applied to the one surface of the piston. In this manner the band brake is released. When shifting from third gear to fourth gear is desired, the piston for fourth gear, which can operate independently of the piston for second gear, is applied with a hydraulic pressure to apply the band brake. Thus, by applying a hydraulic pressure to these two pistons in the above-mentioned manner, shifting from second to third gear and from third to fourth gear is carried out.
However, when the above-mentioned type servo device is applied to automatic transmissions of a type which, for achieving for example light weight and compact construction, has no forward one-way clutch, it becomes very difficult to adjust the gear shift timing from third to fourth gear.
In the automatic transmission disclosed by the above-mentioned publication, a forward one-way clutch is employed, which is engaged in forward gears other than fourth gear. That is, when, with a low clutch (which is engaged in first, second or third gear) kept engaged, a hydraulic pressure for fourth gear is applied to the servo device, the band brake is applied and thus fourth gear is established in the transmission.
However, in a transmission having no forward one-way clutch, it is necessary to release the low clutch when shifting from third to fourth gear. That is, when shifting from third to fourth gear, release of the low clutch and engagement of the band brake are both necessary. In other words, both "discharging" of a hydraulic pressure from the low clutch and "applying" of a hydraulic pressure to one piston for the band brake must be carried out in a well timed manner. However, as is known, under such a condition, it is very difficult to obtain well timed operations of the low clutch and the band brake (viz., piston).